1. Field of the Invention
The present invention pertains to backlight displays and more particularly to wide-range dual-level lighting for backlight displays for avionics applications.
2. Discussion of Related Art
Avionics Liquid crystal display (LCD) units used for avionic applications are often provided with two light-intensity levels. A first, higher intensity level is used for direct backlighting for daylight viewing conditions. A second, lower intensity level just bright enough for viewing at night, is employed when the user's eyes are fully dark-adapted.
U.S. Pat. No. 5,211,463 issued May 18, 1993 to Kalmanash entitled BACKLIGHT FOR LIQUID CRYSTAL DEVICES and U.S. Pat. No. 5,143,433 issued Sep. 1, 1992 to Farrell entitled NIGHT VISION BACKLIGHTING SYSTEM FOR LIQUID CRYSTAL DISPLAYS both disclose systems using a first daylight illumination light source and a second nightlight illumination light source.
Fluorescent lamps are particularly well-suited for providing the backlighting needed for LCD display panels because they can be embodied as long serpentine tubes having a narrow-cross section, providing a thin, even illumination over a wide area.
The electrical characteristics of fluorescent lamps however, present problems in avionics displays, where night vision is critically important and the illumination must be smoothly variable from daylight to nightlight over a wide range of luminance values. When the same fluorescent lamps that are used for higher intensity daylight viewing of LCD displays are also used for lower intensity night lighting applications, the range of usable light-intensity is severely restricted because when operated at low light-intensity night lighting levels, the luminance level of fluorescent lamps is unstable. This is due to electrostatic forces within the lamp that produce a non-uniform glow that varies with time and is unevenly distributed along the lighted arc. To overcome this problem and permit the fluorescent lamps being used to operate within their respective, rather narrow, stable luminance ranges, a separate set of additional lamps is frequently used that is specifically selected to provide the low-level luminance needed for night-time use.
Some problems with the use of separate nightlight fluorescent lamps is that the dimming characteristics of different types of fluorescents differ considerably, they are significantly non-linear, and they change with age, making the coordination of multiple sets of lamps for smooth luminance changes from daylight to nightlight difficult. Also, the operating characteristics of the relatively large lamps used for high-level daylight illumination are temperature sensitive: slow to fire and less stable at lower temperatures than smaller lamps used for nightlighting.
FIG. 1 illustrates a typical prior art scheme for indirect lighting for avionics displays. Prior art embodiments such as disclosed in the aforesaid U.S. Pat. No. 5,211,463 patent use bulky edge-lighted wedges 17 and/or half-silvered mirror surfaces 18 to redirect the light such as shown in FIG. 1. These devices permit the use of smaller night lamps, but result in a loss of energy efficiency in daylight operation and substantial penalties in increased weight and unit cost.
Another aspect of backlight display systems is that they should be compatible with night-vision goggles. Avionics displays must not saturate the high gain image intensifier detectors used by these goggles for night-vision imaging.